U.S. patent application number 09/318482 was filed with the patent office on 2001-08-23 for power amplification using a direct-upconverting quadrature mixer topology.
Invention is credited to EIDSON, DONALD BRIAN, GRANGE, ROBERT EDMUND.
Application Number | 20010016016 09/318482 |
Document ID | / |
Family ID | 23238362 |
Filed Date | 2001-08-23 |
United States Patent
Application |
20010016016 |
Kind Code |
A1 |
EIDSON, DONALD BRIAN ; et
al. |
August 23, 2001 |
POWER AMPLIFICATION USING A DIRECT-UPCONVERTING QUADRATURE MIXER
TOPOLOGY
Abstract
A power amplifier for use in a wireless communication system
operates like a quadrature mixer with conversion gain. The
amplifier includes a local oscillator that generates two reference
signals 90.degree. out of phase with respect to each other. Four
quadrature mixer elements receive signals representing the in-phase
and quadrature components of a baseband signal. Each quadrature
mixer amplifies the in-phase or the quadrature component of the
baseband signal while upconverting with the corresponding reference
signal.
Inventors: |
EIDSON, DONALD BRIAN; (SAN
DIEGO, CA) ; GRANGE, ROBERT EDMUND; (SAN DIEGO,
CA) |
Correspondence
Address: |
BRINKS HOFER GILSON & LIONE
P.O. BOX 10395
CHICAGO
IL
60610
US
|
Family ID: |
23238362 |
Appl. No.: |
09/318482 |
Filed: |
May 25, 1999 |
Current U.S.
Class: |
375/302 |
Current CPC
Class: |
H03C 3/40 20130101; H03D
7/1458 20130101; H03D 7/165 20130101; H03F 3/211 20130101; H04L
27/2071 20130101; H03D 7/1433 20130101; H03D 7/1441 20130101; H03D
2200/0007 20130101 |
Class at
Publication: |
375/302 |
International
Class: |
H03C 003/00; H03K
007/06; H04L 027/12 |
Claims
What is claimed is:
1. A power amplifier comprising: (a) an oscillator circuit
configured to generate two reference signals that are 90.degree.
out of phase with respect to each other; and (b) a quadrature mixer
having: (1) two mixer elements, each connected to the oscillator
circuit to receive one of the reference signals, and each
configured to: (A) receive a signal containing either an in-phase
(I) component or a quadrature (Q) component of a baseband signal to
be transmitted; and (B) use the reference signal to upconvert and
amplify the in-phase or the quadrature component of the baseband
signal; and (2) combining circuitry configured to combine the
upconverted and amplified I and Q components to form an output
signal.
2. The power amplifier of claim 1, wherein the reference signals
have power levels much greater than the power levels of the I and Q
components of the baseband signal.
3. The power amplifier of claim 1, wherein at least one of the
mixer elements includes a power transistor that receives one of the
reference signals as input.
4. The power amplifier of claim 3, wherein the quadrature mixer
includes a Gilbert cell.
5. The power amplifier of claim 3, wherein the power transistor
includes a dual-gate FET.
6. The power amplifier of claim 3, wherein at least one of the
mixer elements has a modulating port driven by the reference signal
and a reference port driven by the in-phase or quadrature component
of the baseband signal.
7. The power amplifier of claim 6, wherein the reference port is
biased to act as an ON/OFF switch.
8. The power amplifier of claim 7, wherein the ON/OFF switch is a
power device.
9. The power amplifier of claim 7, wherein the modulating port is
biased to act as a current source.
10. The power amplifier of claim 1, wherein each of the mixer
elements is configured to operate as a controlled current source
coupled to an ON/OFF switch.
11. The power amplifier of claim 10, wherein the ON/OFF switch is a
power device.
12. The power amplifier of claim 1, comprising four mixer elements,
each of which is configured to receive one of a group of signals
representing the in-phase, negative in-phase, quadrature, and
negative quadrature components of the baseband signal.
13. The power amplifier of claim 12, wherein the oscillator circuit
is configured to produce four reference signals, three of which are
90.degree., 180.degree., and 270.degree. out of phase with respect
to the other.
14. The power amplifier of claim 13, wherein the oscillator circuit
includes a four-phase, {fraction (1/4)}-wavelength branch tap to
generate the four reference signals.
15. A wireless transmitter comprising: (a) a baseband encoder that
receives digital information and produces signals representing an
in-phase (I) component and a quadrature (Q) component of a baseband
signal to be transmitted; (b) an oscillator circuit configured to
generate two reference signals that are 90.degree. out of phase
with respect to each other; and (c) a quadrature mixer having: (1)
two mixer elements, each connected to the oscillator circuit to
receive one of the reference signals and to the baseband encoder to
receive one of the components of the baseband signal, and each
configured to use the reference signal to upconvert and amplify the
in-phase or the quadrature component of the baseband signal; and
(2) combining circuitry configured to combine the upconverted and
amplified I and Q components to form an output signal.
16. The wireless transmitter of claim 15, wherein the reference
signals have power levels much greater than the power levels of the
I and Q components of the baseband signal.
17. The wireless transmitter of claim 15, wherein at least one of
the mixer elements includes a power transistor that receives one of
the reference signals as input.
18. The wireless transmitter of claim 17, wherein the quadrature
mixer includes a Gilbert cell.
19. The wireless transmitter of claim 17, wherein the power
transistor includes a dual-gate FET.
20. The wireless transmitter of claim 17, wherein at least one of
the mixer elements has a modulating port driven by the reference
signal and a reference port driven by the in-phase or quadrature
component of the baseband signal.
21. The wireless transmitter of claim 20, wherein the reference
port is biased to act as an ON/OFF switch.
22. The wireless transmitter of claim 21, wherein the modulating
port is biased to act as a current source.
23. The wireless transmitter of claim 15, further comprising an
element configured to produce four baseband signals representing
the in-phase and inverted in-phase components and the quadrature
and inverted quadrature components of the baseband signal.
24. The wireless transmitter of claim 23, comprising four mixer
elements, each of which is configured to receive one of the four
baseband signals.
25. The wireless transmitter of claim 23, wherein the oscillator
circuit is configured to produce four reference signals, three of
which are 90.degree., 180.degree., and 270.degree. out of phase
with respect to the other.
26. The wireless transmitter of claim 25, wherein the oscillator
circuit includes a four-phase, {fraction (1/4)}-wavelength branch
tap to generate the four reference signals.
27. The wireless transmitter of claim 15, further comprising a
low-loss combiner coupled to the quadrature mixer elements.
28. The wireless transmitter of claim 27, wherein the low-loss
combiner includes a ceramic substrate.
29. A method for use in amplifying a baseband signal to be
transmitted over a wireless network, the method comprising: (a)
providing two reference signals that are 90.degree. out of phase
with respect to each other to a quadrature mixer; (b) providing an
in-phase (I) and quadrature (Q) component of the baseband signal to
the quadrature mixer; (c) operating the quadrature mixer such that
the reference signals upconvert and amplify the in-phase and the
quadrature components of the baseband signal; and (d) combining the
upconverted and amplified I and Q components to form an output
signal.
30. The method of claim 29, comprising providing at least one of
the reference signals to a power transistor in the quadrature
mixer.
31. The method of claim 30, wherein providing at least one of the
reference signals to a power transistor includes providing the
reference signal to a dual-gate FET.
32. The method of claim 30, further comprising driving a modulating
port of the quadrature mixer with one of the reference signals and
driving a reference port with either the in-phase or the quadrature
component of the baseband signal.
33. The method of claim 32, further comprising biasing the
reference port to act as an ON/OFF switch.
34. The method of claim 32, further comprising biasing the
modulating port to act as a variable current source.
35. The method of claim 29, wherein the quadrature mixer includes
four mixer elements.
36. The method of claim 35, comprising providing one of a group of
signals to each of the mixer elements, where the group includes
signals representing the in-phase and inverted in-phase components
and the quadrature and inverted quadrature components of the
baseband signal.
37. The method of claim 36, comprising providing one of four
reference signals to each of the mixer elements, where three of the
reference signals are 90.degree., 180.degree., and 270.degree. out
of phase with respect to another of the reference signals.
38. The method of claim 37, further comprising using a four-phase,
{fraction (1/4)}-wavelength branch tap to generate the reference
signals.
Description
TECHNICAL FIELD
[0001] This invention relates to wireless communications and, more
particularly, to power amplification using a direct-upconverting
quadrature mixer topology.
BACKGROUND
[0002] A conventional power amplifier (PA) in a digital wireless
transceiver receives and then amplifies a radio frequency (RF)
signal. For many modulation schemes, including those that comply
with the IS-95, PDC, PHS, DCT and certain CDMA standards, the RF
signal has a non-constant envelope, which requires the power
amplifier to operate with a high degree of linearity over a large
dynamic range. As a result, the conventional power amplifier
usually is biased for operation between the Class A and Class B
modes. The best case (asymptotic) efficiency of a PA operating in
this manner usually lies between 50% and 78% for constant envelope
signals; the efficiency is even lower for non-constant envelope
signals.
[0003] U.S. patent application Ser. No. 09/108,628, filed on Jul.
1, 1998, by Donald Brian Eidson and Robert Edmund Grange, and
titled "Envelope Feedforward Technique with Power Control for
Efficient Linear RF Power Amplification," discloses a power
amplification technique that allows operation in high efficiency
modes (such as Class D or Class E), in some cases producing
asymptotic efficiencies for constant envelope signals that approach
100%. In one implementation of this technique, a dual-gate field
effect transistor (FET) acts in a manner similar to a conventional
RF mixer, receiving at one dual-gate input a constant envelope RF
signal containing phase component information, and receiving at the
other dual-gate input an unmodulated signal containing the envelope
component. In a digital system, the constant envelope RF signal and
the unmodulated envelope signal are derived from the in-phase (I)
and quadrature (Q) components provided by a baseband device.
SUMMARY
[0004] In one aspect, the invention features a power amplifier
having a quadrature mixer and an oscillator circuit. The oscillator
circuit generates two reference signals that are 90.degree. out of
phase with respect to each other. The quadrature mixer has two
mixer elements that each receive one of the reference signals. Each
mixer element receives a signal containing either the in-phase or
the quadrature component of a baseband signal to be transmitted and
uses the reference signal to upconvert and amplify the I or Q
component of the baseband signal. The quadrature mixer also
includes circuitry that combines the upconverted and amplified I
and Q components to form an output signal.
[0005] In another aspect, the invention features a wireless
transmitter that includes a baseband encoder, an oscillator
circuit, and a quadrature mixer. The baseband encoder produces
signals representing the in-phase and quadrature components of a
baseband signal to be transmitted. The oscillator generates two
reference signals that are 90.degree. out of phase with respect to
each other. The quadrature mixer uses each of the reference signals
to upconvert and amplify one of the in-phase and quadrature
components of the baseband signal.
[0006] In some embodiments, the reference signals have power levels
much greater than the power levels of the I and Q components of the
baseband signal. In these cases, the mixer elements include power
elements, such as Gilbert cells or dual-gate FETs. The reference
signal often drives a modulating port of the mixer, and the
in-phase or quadrature component of the baseband signal often
drives the reference port. In some cases, the reference port is
biased to act as an ON/OFF switch, and the modulating port is
biased to act as a current source. The ON/OFF switch is usually
implemented as a power device.
[0007] Other embodiments include four mixer elements, each of which
receives one of a group of signals representing the in-phase,
negative in-phase, quadrature, and negative quadrature components
of the baseband signal. In some of these embodiments, the
oscillator circuit produces four reference signals, three of which
are 90.degree., 180.degree., and 270.degree. out of phase with
respect to the other. A four-phase, {fraction (1/4)}-wavelength
branch tap is one tool for producing the four reference
signals.
[0008] Other embodiments include a low-loss combiner coupled to the
quadrature mixer elements. In some of these embodiments, the
low-loss combiner includes a ceramic substrate.
[0009] The details of one or more embodiments of the invention are
set forth in the accompanying drawings and the description below.
Other features, objects, and advantages of the invention will be
apparent from the description and drawings, and from the
claims.
DESCRIPTION OF DRAWINGS
[0010] FIG. 1 is a block diagram of a power amplifier that uses a
quadrature mixer topology to amplify the in-phase (I) and
quadrature (Q) components of a baseband signal.
[0011] FIG. 2 is block diagram of an alternative power amplifier
that uses a quadrature mixer topology to amplify the in-phase (I)
and quadrature (Q) components of a baseband signal.
[0012] FIG. 3 is a schematic diagram of a pair of dual-gate field
effect transistors (FETs) used to implement the quadrature mixer
topology of FIG. 2.
[0013] FIG. 4 is a diagram illustrating the operation of the
dual-gate FETs in the arrangement of FIG. 3.
[0014] Like reference numbers and designations in the various
drawings indicate like elements.
DETAILED DESCRIPTION
[0015] The power amplification technique described here uses a
quadrature mixer topology to amplify the in-phase and quadrature
components of a signal to be transmitted. This technique allows
operation in high efficiency modes, which leads to improvements in
efficiency over Class A amplifiers by a factor of two and over
Class B amplifiers by a factor of 4/.pi.. Because the envelope and
phase information need not be derived from the in-phase and
quadrature components, this technique can be used with virtually
any manufacturer's baseband circuitry for virtually any
communication standard.
[0016] Through direct upconversion, this technique eliminates
intermediate frequency (IF) and radio frequency (RF) circuitry in
the transmit chain, which reduces the overall cost and complexity
of a transmitter solution. This technique also facilitates factory
tuning, since only the power amplifier (PA), and not an IF
quadrature mixer, need be tuned. However, this technique differs
from conventional IF mixing techniques in several ways. For
example, this technique allows harmonic matching of the combiner
output, as one does with a power amplifier and not a conventional
mixer. This reduces the square-wave-to-sinusoidal output conversion
loss associated with the mixer. Also, this technique allows
cascaded gain stages where only one of the stages (e.g., the last
stage) performs the mixing function.
[0017] This technique also combines high-power inputs with
low-power I and Q inputs. The input impedance of an IF power
amplifier at high frequencies is relatively low (typically 50
.OMEGA.) and thus requires a large drive current to operate. In
contrast, the impedance seen by the I and Q inputs is relatively
high because, in general, the I and Q signals contain only low
frequency content. Therefore, the I and Q inputs do not require as
much drive current to operate the devices that they drive. In
contrast, conventional mixers tend to have IF and I-Q input
impedances that are much more comparable to each other (often on
the order of 1 k.OMEGA.).
[0018] FIG. 1 shows a wireless transceiver 10 having a power
amplifier (PA) 12 arranged in a quadrature mixer, or I-Q modulator,
configuration. Like a conventional transceiver, this transceiver 10
includes a baseband encoder 14 that receives digital information
and produces signals representing the in-phase (I) and quadrature
(Q) components of the baseband signal to be transmitted. The
in-phase and quadrature signals pass through digital-to-analog
converters 16, 18 and gain control elements 20, 22, as they would
in a conventional transceiver, in arriving at the power amplifier
12. A local oscillator 28 generates a reference signal that serves
as one component (cos .omega..sub.ct) of a carrier signal at a
frequency .omega..sub.c. A phase delay element 30, such as a
.lambda./4 branch tap, provides another reference signal that is
90.degree. out of phase with the first reference signal. The second
reference signal serves as another component (sin .omega..sub.ct)
of the carrier signal. In most cases, the power amplifier 12
includes cascaded gain elements 25A, 25B, 35A, 35B that amplify the
reference signals before providing the reference signals to mixer
elements 24, 26 (described below). These cascaded gain elements
provide additional gain to the power amplifier 12. For example,
many mixer configurations provide only 10 dB of gain. Adding two
cascaded gain elements 25A, 25B, 35A, 35B in the path of each
reference signal produces additional gain on the order of 20
dB.
[0019] The power amplifier 12 includes two mixer elements 24, 26
and a signal combiner 32 that receive the in-phase and quadrature
components of the baseband signal and the carrier signal and form
an amplified RF signal. In many embodiments, each of the mixer
elements 24, 26 is implemented as a dual-gate field effect
transistor (FET), one gate of which (the upper gate) receives
either the in-phase or the quadrature component of the baseband
signal as input, and the other gate of which (the lower gate)
receives the corresponding component of the carrier signal. In this
configuration, each dual-gate FET imparts gain to the baseband
signal while modulating the signal onto the carrier wave.
[0020] In one implementation, each dual-gate FET is biased to
operate in the same manner as a conventional single-ended
quadrature mixer with conversion gain. The lower gate of the
dual-gate FET is biased to operate as a "+1/-1 switch," and the
upper gate receives either the I or Q input. The top gate of each
dual-gate FET is biased to support both positive and negative
voltage swings of the in-phase or quadrature component of the data
signal. In another implementation, the lower gate is biased to
operate as an ON/OFF switch and the upper gate receives a bipolar I
or Q input. A harmonic circuit on the output then recreates the
waveform components which ordinarily would result from the now
nonexistent "-1 swing" of the lower gate. In both of these
implementations, the individual mixer stages would behave like
Class A amplifiers if both negative and positive (bipolar) I and Q
values were supported. In order to obtain true high efficiency
operation (e.g., Class D, E, or F operation), each mixer supports
only positive voltages.
[0021] The signal combiner 32 combines the modulated and amplified
I and Q signals to form an output signal. The signal combiner 32
delivers the output signal to a PA matching network 38.
[0022] FIGS. 2 and 3 show a more efficient implementation of the
power amplifier 12, in which the quadrature mixer topology is
carried out by four dual-gate FETs 40, 42, 44, 46 arranged in a
push-pull configuration. In this configuration, two
digital-to-analog converters (DACs) 45, 47 produce four
differential inputs (I.sup.+, I.sup.-, Q.sup.+, Q.sup.-)
representing direct and inverted versions of the in-phase and
quadrature components of the baseband signal. A local oscillator 48
produces a reference signal representing the in-phase component
(cos .omega..sub.ct) of the carrier waveform. A multiple phase
delay element 49, such as a four-phase .lambda./4 branch tap,
produces an inverted version of the reference signal (-cos
.omega..sub.ct), as well as two other signals representing the
direct and inverted quadrature components (sin .omega..sub.ct and
-sin .omega..sub.ct) of the carrier waveform. In many cases, gain
control elements 50, 52, 54, 56 are used to amplify the
differential baseband signals before they reach the power
amplifier. One or more cascaded gain elements 43 also can be used
to amplify the reference signals.
[0023] Each dual-gate FET 40, 42, 44, 46 receives one of the
baseband signal components (I.sup.+, I.sup.-, Q.sup.+, Q.sup.-) at
its upper dual-gate port and the corresponding component of the
carrier waveform at its lower dual-gate port. As a result, each
dual-gate FET 40, 42, 44, 46 handles only positive voltages, which
allows the use of n-channel devices in implementing the dual-gate
FETs. A low-loss combiner 55 combines the output signals from the
dual-gate FETs 40, 42, 44, 46. Implementing the combiner with a
ceramic substrate improves the performance of the PA.
[0024] FIG. 4 shows an implementation in which each of the
dual-gate FETs operates as a controlled current source 60 coupled
to a single-pole, single-throw (SPST) switch 62. The sinusoidal
waveform at the lower gate drives the switch, closing the switch
and thus driving the output line 54 only when the waveform has a
positive value. The value of the differential baseband signal at
the upper gate governs the amount of current flowing through the
current source 60. In this configuration, the power amplifier
operates in a high efficiency mode.
[0025] A number of embodiments of the invention have been
described. Nevertheless, it will be understood that various
modifications may be made without departing from the spirit and
scope of the invention. For example, while the invention has been
described in terms of dual-gate FETs arranged in a quadrature mixer
topology, other embodiments use devices such as bipolar transistors
and HEMTs arranged in push-pull configurations and modeled with
equivalent circuits like that shown in FIG. 4. Gilbert cell mixers
with RF inputs optimized for power (i.e., with low input
impedances) are used in some embodiments. In some embodiments, the
gain control elements are used to amplify signals other than the
baseband signals, such as the carrier signal produced by the local
oscillator. Many embodiments include cascaded gain control elements
to further increase power control. Accordingly, other embodiments
are within the scope of the following claims.
* * * * *